1. Field
The invention relates to a nickel/iron-based braze and a process for brazing two or more parts.
2. Description of Related Art
Soldering is a process for joining metal or ceramic parts using a molten filler material known as solder. A distinction is drawn between soft soldering and brazing (hard soldering) on the basis of the working temperature of the solder, said working temperature typically lying 10° C. to 60° C. above the liquidus temperature of the solder. Soft solders are worked at temperatures below 450° C., whilst brazes are worked at temperatures above 450° C. Brazes are used in applications in which high mechanical stability of the soldered joint and/or high mechanical stability at high operating temperatures are desired.
Brazes have been typically worked at temperatures of approximately 1200° C. In the case of certain parent metals (i.e., the metals being joined together) however, efforts are frequently made to achieve a lower soldering/working temperature for the braze in order to avoid temperature-induced changes in the parent metal.
For example, in the case of steels, coarse grain formation commences at a temperature of 1000° C. and increases significantly as this temperature rises further. Such coarse grain formation is undesirable as it leads to a significant reduction in the mechanical stability of the parent metal.
A low soldering temperature is also desirable in the brazing of precipitation-hardened Ni-based alloys since, in addition to considerable grain coarsening, working temperatures above approximately 1050° C. also lead to an irreversible deterioration in stress rupture strength which cannot be remedied by further heat treatment.
It is also desirable to be able to produce the brazes in various forms such as solder paste and ductile foils, for example, thereby extending the range of application of the brazes.
Certain nickel/iron/chromium-based braze pastes are disclosed in U.S. Pat. No. 4,402,742, for example. However, the liquidus temperatures of these brazes are well above 1000° C. The working temperature is 10° C. to 60° C. above these temperatures and is therefore too hot for certain parent metals. Moreover, the total metalloid content of B and Si is high, and these alloys cannot therefore be produced as ductile foils.
It is therefore desirable to have a nickel-based braze which can be produced in the form of both a solder paste and a ductile foil.